The present invention relates to color picture tubes having improved inline electron guns, and particularly to such guns having an improved expanded focus lens for reduced spherical aberration.
An inline electron gun is one designed to generate or initiate preferably three electron beams in a common plane and direct those beams along convergent paths in that plane to a point or small area of convergence near the tube screen. In one type of inline electron gun shown in U.S. Pat. No. 3,873,879, issued to R. H. Hughes on Mar. 25, 1975, the main electrostatic focusing lenses for focusing the electron beams are formed between two electrodes referred to as the first and second accelerating and focusing electrodes. These electrodes include two cup-shaped members having bottoms facing each other. Three apertures are included in each cup bottom to permit passage of three electron beams and to form three separate main focus lenses, one for each electron beam. In a preferred embodiment, the overall diameter of the electron gun is such that the gun will fit into a 29 mm tube neck. Because of this size requirement, the three focusing lenses are very closely spaced from each other, thereby providing a severe limitation on focus lens design. It is known in the art that the larger the focus lens diameter, the less will be the spherical aberration which restricts the focus quality.
In addition to the focus lens diameter, the spacing between focus lens electrode surfaces is important, because greater spacing provides a more gentle voltage gradient in the lens which also reduces spherical aberration. Unfortunately, greater spacing between electrodes beyond a particular limit (typically 1.27 mm) generally is not permissible because of beam bending from electrostatic charges on the neck glass penetrating into the space between the electrodes, which causes electron beam misconvergence.
In copending U.S. patent application Ser. No. 201,692, filed Oct. 29, 1980 by R. H. Hughes and B. G. Marks, now Pat. No. 4,370,592, an electron gun is described wherein the main focus lens is formed by two spaced electrodes. Each electrode includes a plurality of apertures therein equal to the number of electron beams and also a peripheral rim, with the peripheral rims of the two electrodes facing each other. The apertured portion of each electrode is located within a recess set back from the rim. The effect of this main focus lens is to provide the gentle voltage gradient sought to reduce spherical aberration. Because of the asymmetrical shape of the peripheral rims of the two electrodes, described in patent application Ser. No. 201,692, horizontal and vertical focus voltage components for the inner and outer guns are not the same. In the vertical direction, the center electron beam sees more a slot, and experiences more focusing action, than the sides, where the focusing geometry is bounded in part by a circular arc. This is because the field penetrates the slot in the vertical direction more easily than an inscribed circular boundary. Likewise, the horizontal focusing component at the outer electron beams may be more active than at the center beam, because the field in the horizontal direction falls away more rapidly at the side ends of the peripheral rims than within the center of the recessed cavity. Therefore, there is a need to modify the peripheral rim geometry to unitize the focus voltages.